MRI is an imaging method which excites nuclear spin of an object set in a static magnetic field with a RF (radio frequency) signal having the Larmor frequency magnetically and reconstructs an image based on MR (magnetic resonance) signals generated due to the excitation.
In MRI, gradient magnetic fields are applied by gradient coils for acquiring MR signals. However, gradient magnetic fields are generated as pulse waves. Therefore, in case of being an electrical conductor around a gradient coil, an eddy current is generated in the electrical conductor when a gradient magnetic field rises and falls.
A heat shield of a static field magnet is included in examples of electrical conductors. When a super conducting magnet which generates a static magnetic field not less than 0.5 T is used as a static field magnet, a metallic container including liquid helium is provided as a heat shield in the super conducting magnet. Additionally, plural metallic containers such as a metallic container including liquid nitrogen are arranged around the liquid helium layer. Therefore, applying a gradient magnetic field produces an eddy current in each metallic container.
Temperatures, materials and sizes of respective metallic containers set in a static field magnet are different mutually. Therefore, an intensity and an attenuation time constant of eddy current generated in each metallic container has plural components. Generally, a time constant of an eddy current is in a wide range from 0.2 ms to 3 ms.
Meanwhile, an application of a gradient magnetic field also produces a self-eddy current in a gradient magnetic field coil material itself. The self-eddy current sometimes produces considerable strain of a magnetic field.
The eddy current as mentioned above produces an eddy magnetic field which changes due to the eddy current and generates a strain in a waveform of a gradient magnetic field outputted as a controlling value from a controller in a MRI apparatus. Then, the strain of the gradient magnetic field leads to an image artifact.
Accordingly, an Actively Shielded Gradient Coil (ASGC) to suppress generation of an eddy magnetic field is devised. Alternatively, compensation of an eddy magnetic field which corrects a waveform of a gradient magnetic field strained by an eddy magnetic field is devised. In principle, ASGC makes it possible to reduce an intensity of an eddy magnetic field substantially.
However, practically, it is not possible to prevent a minute eddy magnetic field from being generated for reasons such as production error of an ASGC and discrete arrangement of coil wires. Therefore, in the case of using a high-speed imaging method such as an EPI (echo planar imaging) method, it is possible to generate artifact in an image by the presence of a slight eddy magnetic field. Then, it is preferable to perform compensation of an eddy magnetic field even if a gradient magnetic field is applied with an ASGC.
The method to adjust a waveform of a gradient magnetic field set as a pulse sequence so as to cancel an eddy magnetic field is devised as another technology of suppressing an eddy magnetic field. For example, DWI (diffusion weighted imaging) is performed by an EPI sequence while applying an MPG (motion probing gradient) pulse. The MPG pulse is an intensive gradient magnetic field pulse. Therefore, a technology to adjust a gradient magnetic field other than an MPG pulse in an EPI sequence so as to cancel an eddy magnetic field generated due to the MPG pulse is suggested.
It is significant to measure intensities, time constants and a spatial distribution of eddy magnetic fields in advance with satisfactory accuracy in order to perform compensation of the eddy magnetic fields precisely. For example, in the case of performing DWI, it is significant to measure an eddy magnetic field having a time constant from 0.2 ms to 30 ms with satisfactory accuracy. Intensities and time constants of eddy magnetic fields can be obtained in accordance with phase shift information of MR signals acquired by a pulse sequence for measuring the eddy magnetic fields.
On the other hand, recently, an MRI apparatus which can generate static magnetic field intensity not less than 3 T becomes widely used. Under the high magnetic field as mentioned above, an influence to attenuation of a MR signal intensity by transverse relaxation star (T2*) relaxation may be not negligible. That is, both phase shifts by eddy magnetic fields and T2* attenuation occur in MR signals. In this case, it becomes difficult to obtain intensities and time constants of eddy magnetic fields from phase shift amounts of MR signals precisely. Especially, when DWI is performed, it becomes more difficult to measure intensities and time constants of eddy magnetic fields with high accuracy since a time constant of an eddy magnetic field becomes equivalent to that of T2* attenuation.
That is, it is difficult to measure intensities and time constants of eddy magnetic fields, each having a time constant from 0.2 ms to 30 which is equivalent to that of T2* attenuation especially, with satisfactory accuracy under a high magnetic field not less than 3 T with a conventional technology. Not only under a high magnetic field, it is desired to measure intensities and time constants of eddy magnetic fields with satisfactory accuracy.